1. Field of the Invention
This invention relates to a process for continuously freeing polyolefins from residual hydrocarbons.
2. Description of the Prior Art
It is known that .alpha.-olefins or mixtures thereof can be polymerized by the Zeigler low pressure process with the aid of catalysts, which contain compounds of the elements belonging to subgroups IV to VI of the periodic system in combination with organometal compounds of the elements belonging to groups I to III of the periodic system, in saturated liquid C.sub.6 -C.sub.11 hydrocarbons or suitable mixtures thereof, e.g. gasoline. During the work-up of the resulting polymers, the polyolefins which are suspended in the gasoline are separated from the dispersant by filtration with the use of a filter press. The polyolefins which are moist with gasoline can then be freed in various manner from adhering dispersant, e.g. by directly drying them in a contact drier or with the aid of hot air or an inert gas or by intensive steam distillation (cf. German Patent Specification "Auslegeschrift" 1 089 547).
Experience has shown that it is technically good practice to disperse gasoline-containing polymers in water and separate the gasoline from the aqueous phase by distillation with agitation, in the form of an azeotropic mixture with water. This method is not fully satisfactory, however, as considerable energy is required to be used for the distillative removal of residual gasoline. In addition to this, the distillative treatment gives rise to the formation of widely different azeotropic gasoline/water-mixtures, which call for the use of specifically designed distilling columns of appropriate separating efficiency.
A further process for removing monomers from an aqueous dispersion of a polymer containing at least 50 weight% of polymerized vinyl chloride has been described in German Patent Specification "Offenlegungsschrift" 2 521 780, wherein the dispersion is introduced into the upper portion of a sieve plate column and contacted countercurrently therein over a period of about 10 seconds to 20 minutes with steam of about 100.degree. to 150.degree. C. under a pressure of about 600 to 1200 mm Hg. Next, the polymer dispersion is removed through the base portion of the column, while a vapor mixture which escapes overhead is subjected to stagewise condensation so as to recover an aqueous phase and the monomers.
The process just described is not suitable for use in the removal of residual hydrocarbons from polyolefins, for the following reasons:
As compared with polyolefins, polyvinyl chloride has a density of more than 1.4 i.e. a density higher than that of water. In other words, an aqueous polyvinyl chloride dispersion which is introduced into the upper portion of a column will naturally travel downwardly, countercurrently to steam which ascends therein. This flow behaviour of polyvinyl chloride is in contrast with that of polyolefins which are specifically less dense than water. They float on the water surface rather than to travel downwardly in a column. It is also noted that the process described in German Patent Specification "Offenlegungsschrift" 2 521 780 is used for the removal of vinyl chloride which is gaseous under standard conditions, whereas the residual hydrocarbons which are to be removed from polyolefins have a boiling range of 60.degree. to 190.degree. C. and are accordingly in the liquid state. It should also be borne in mind that an aqueous polyvinyl chloride dispersion contains vinyl chloride in proportions of some ppm (ppm stands for parts per million) which do not substantially affect the nature of the dispersion. In contrast with this, the polyolefin dispersions which are to be treated in accordance with this invention contains considerably more significant proportions of undesirable materials which may (a) seriously affect the interfacial activity and are (b) liable to form azeotropes with the dispersant. Needless to say therefore the removal of gasoline raises a problem in respect of diffusion and entails additional difficulties which arise during the distillation, e.g. the deposition of solid matter or formation of foam.